This invention relates to a field effect transistor which is operable in a millimeter wave band and which is formed by the use of a compound semiconductor.
In general, it is well known in the art that a semiconductor device formed by silicon can not be operated in such a millimeter wave band. Taking this into consideration, a wide variety of compound semiconductor devices have been proposed in order to enable operation in the millimeter wave band. Among others, recent attention has been directed to a field effect transistor which has a channel formed by a two-dimensional electron gas (2-DEG) developed in a heterojunction structure.
As a rule, a conventional semiconductor device, such as a field effect transistor of a heterojunction structure type, is often implemented by a ternary or a quarternary compound semiconductor, such as InGaAs, InGaAsP. Such a conventional semiconductor device is described by K. H. G. Duh et al in IEEE MICROWAVE AND GUIDED WAVE LETTERS, VOL. 1, NO. 5, (May, 1991) and is entitled "A Super Low-Noise 0.1 .mu.m T-Gate InAlAs-InGaAs-InP HEMT". More specifically, the conventional semiconductor device disclosed above is specified by a sandwiched structure which comprises a semi-insulator substrate of InP, a buffer layer formed by undoped or intrinsic InAlAs on the semi-insulator substrate, a channel layer of undoped InGaAs, a spacer layer formed by undoped InAlAs, a carrier supply layer of InAlAs doped with an n-type impurity, a Schottky layer of undoped InAlAs, and an InGaAs cap layer doped with an n-type impurity. A gate electrode is attached to the Schottky layer while source and drain electrodes are formed on the cap layer. With this structure, the gate electrode is located between the source and the drain electrodes.
With this structure, the channel layer of the undoped InGaAs is interposed between the buffer layer of the undoped InAlAs and the spacer layer of the undoped InAlAs. At any rate, the above-mentioned heterostructure serves to enhance a two-dimensional electron gas density and to reduce series resistance and increase current density. Practically, the field effect transistor achieves a noise figure of 1.2 dB and a gain of 7 dB at a frequency of 94 GHz.
Alternatively, another conventional semiconductor device is also disclosed by G. I. NG in IEEE ELECTRON DEVICE LETTERS, VOL. 10, NO. 3, Page 114 (March, 1989) and entitled "Improved Strained HEMT Characteristics Using Double-Heterojunction In.sub.0.65 Ga.sub.0.35 As/In.sub.0.52 Al.sub.0.48 As Design". The semiconductor device disclosed by NG et al comprises a channel layer of InGaAs which includes an amount of In greater than 0.53 relative to an amount of Ga in order to improve a device characteristic in comparison with the device disclosed by Duh et al. However, it is to be noted that a thickness of the channel layer is limited by an amount of In included in the channel layer because a lattice-mismatch takes place between the semi-insulator substrate of InP and the channel layer of InGaAs when an amount of In exceeds 0.53. As a result, a breakdown voltage is unfavorably reduced in the semiconductor device disclosed by NG.
Furthermore, disclosure is also made by T. Akazaki et al in IEEE ELECTRON DEVICE LETTERS, VOL. 13, NO. 6, Pages 325-327, (June 1992), about another conventional device that comprises a channel layer of InGaAs in which a thin InAs film is inserted.
Thus, each of the conventional devices mentioned above has the channel layer of InGaAs in common.
Herein, it is noted that an effective mass of electrons running or flowing in the channel layer of InGaAs becomes light or small as an amount of In increases in the channel layer. This means that a drift velocity of the electrons becomes fast as the effective mass of the electrons becomes small.
It has been kept in mind that the amount of In in the channel layer is equal to 0.53 so as to lattice match the channel layer with the substrate of InP. However, even when an amount of In exceeds 0.53, it is possible to obtain an excellent crystal with a strain applied to the crystal, if a thickness of the channel layer is selected so that any misfit dislocation does not occur.
As mentioned above, it is effective to increase an amount of In within a range such that no misfit dislocation takes place, so as to reduce the effective mass of channel electrons.
Practically, a critical thickness of the channel layer of InGaAs necessary for preventing the misfit dislocation becomes thin with an increase of an amount of In. This makes it difficult to obtain an enough sheet carrier density.
Moreover, a bandgap in the channel layer becomes narrow as an amount of In increases, which unfavorably increases a probability of impact ionization under a high electric field and makes a high speed operation difficult. Accordingly, an increase of In is not preferable when the semiconductor device is operated under the high electric field.
In addition, a structure which has the InAs layer inserted in the channel layer, as proposed by Akazaki et al, is liable to give rise to impact ionization when a high electric field is impressed. This is because electrons are rendered into hot electrons in the InAs layer on impression of the high electric field. Such impact ionization brings about an undesirable increase of a drain conductance and so on and results in deterioration of device performance.
At any rate, it is difficult to stably form a two-dimensional electron gas channel without a misfit dislocation when the channel layer of InGaAs is used in the above-mentioned manners.
Moreover, although the electrons in the channel layer of InGaAs can accomplish a high velocity operation within a low electric field in comparison with electrons included in a channel layer of GaAs, the former can not accomplish a high velocity operation within a high electric field which is higher than about 10 kV/cm because the electrons in the channel layer of InGaAs moves at a drift velocity which is not greatly changed from a drift velocity of the electrons in the GaAs layer. Eventually, use of the channel layer of InGaAs can not assure the high velocity operation within the high electric field as compared with use of the channel layer of GaAs. This makes use of the InGaAs meaningless because the greatest merit of using the InGaAs resides in improvement of the channel electron velocity.